1. Field of the Invention
This invention relates generally to actively controlled generators, and more particularly to an actively controlled alternator for a motor vehicle having a battery and auxiliary electronic loads, which require varying levels of electronic power to be delivered from the alternator and the battery at a set voltage level.
2. Background
A conventional motor vehicle alternator includes a three-phase stator winding, and a rotor magnetically coupled to the stator and driven by the motor vehicle's engine. As the rotor is driven, three-phase power is generated at output connections of the stator winding and rectified by a three-phase diode rectifier bridge, which is traditionally a full-wave diode rectifier bridge.
Generally, the full-wave diode rectifier bridge limits the output power of the conventional motor vehicle alternator, especially at idle speed, which may be characterized as approximately 1500 rpm and 150 Hz electrical frequency.
This is because the natural commutation of diodes included in the full-wave diode rectifier bridge typically forces the phase angle between phase currents and phase voltages at the output connections of the stator winding to be equal to 0 radians, thereby restricting the magnitude of the phase currents of the stator winding and therefore limiting the alternator output power.
This limitation in the alternator output power may be overcome by introducing active switching elements in place of at least some of the diodes in the full-wave diode rectifier bridge. For example, the active switching elements can be activated and deactivated for adjusting the phase angle between the phase currents and the phase voltages of the stator winding. In this way, the phase angle between the phase voltages and a back electromagnetic force (EMF) generated in the phases of the stator winding can also be adjusted, thereby increasing both the magnitude of the phase currents of the stator winding and the alternator output power.
One difficulty in implementing such an active rectifier bridge is in determining the timing of the active switching elements relative to the position and the flux of the rotor. Prior alternator systems have included various combinations of position, voltage, and/or current sensors for determining the timing of the active switches.
For example, in U.S. Pat. No. 5,793,167 ("the '167 patent") issued Aug. 11, 1998, to Liang et al., a system for increasing alternator output power is described. In accordance with that disclosure, the conventional full-wave diode rectifier bridge is replaced with a full-wave controlled rectifier bridge having controlled switches in place of diodes. The rectifier bridge is controlled in response to a third harmonic of the voltage generated by the alternator, thereby synchronizing the rectifier bridge with the alternator. The alternator includes a rotor having a field winding that receives a field current, which is controlled up to a maximum value for partially controlling the alternator output power. The power produced by the alternator is also partially controlled by introducing a phase angle between phase voltages at the three output connections of the stator winding and the third harmonic, up to a maximum or optimum phase angle. In order to increase the alternator output power, the field current is increased up to the maximum value before any phase angle is introduced between the phase voltages and the third harmonic. Similarly, in order to decrease the alternator output power, the phase angle is reduced to 0 radians before the field current is reduced.
However, the system for increasing alternator output power described in the '167 patent has some drawbacks. For example, that system requires a third harmonic detector circuit for combining the three phase voltages and the neutral N from the stator winding, thereby generating a third harmonic within the voltage generated by the alternator. The third harmonic must then be further processed for generating a fundamental frequency output pulse for each of six pulses of the third harmonic voltage signal. The electronics required for implementing the third harmonic detector and for subsequently processing the third harmonic significantly increase the overall cost and complexity of the system.
In addition, in U.S. Pat. No. 5,648,705 ("the '705 patent") issued Jul. 15, 1997, to Sitar et al., a system for increasing alternator output power at low speeds using a controlled, rectifier bridge is described. In accordance with that disclosure, both a current detector and a voltage detector are used for measuring the state of one phase of the three-phase stator winding; and, this information is passed on to a controller, which uses the information to control a full-wave controlled rectifier bridge for optimizing the phase shift between a back EMF in the stator winding and phase voltages at the three output connections of the stator winding. The function of the system described in the '705 patent can be understood by referring to FIG. 5 of the '705 patent. That drawing describes a prior art Lundell alternator including a conventional diode rectifier. For the Lundell alternator such as is commonly used in motor vehicles, the power angle, .beta., represents the angle between the back EMF and the phase voltage, V. As shown in FIG. 5 of the '705 patent, a conventional diode rectifier bridge has an angle of 0 radians between a phase current, I, and the phase voltage, V. The resulting power angle, .beta., is therefore less than .pi./2 radians, which results in a low output power.
Accordingly, it is desirable to increase the power angle, .beta.. To avoid the use of rotation sensors, and because the back EMF cannot be directly measured during loading of the alternator because it cannot be electromagnetically separated from the armature current (reaction), the system of the '705 patent measures both the current and the voltage of a single phase of the stator winding. Using these measurements, the synchronous frequency of the alternator can be determined, thereby determining certain information about the back EMF. This information can then be used to maintain a phase shift, .A-inverted., between the phase current and its corresponding phase voltage to maximize output power, as shown in FIG. 4 of the '705 patent. The phase shift, .A-inverted., can be induced by controlling the switching of the rectifier bridge. The control strategy described in the '705 patent uses a phase current detector and a phase voltage detector to estimate the position of the back EMF from the determined phase current and phase voltage. This estimated position provides an existing delay angle between the phase current and the phase voltage; and, this existing delay angle is compared with a desired delay angle read from experimentally determined optimal values in a look-up table. The switching of the rectifier bridge is then controlled for matching the existing delay angle with the desired delay angle.
However, the system for increasing alternator output power described in the '705 patent also has the drawback that both current and voltage must be measured, thereby requiring two sensors and electronics for gathering and processing the two measurements, and therefore increasing the cost and complexity of the system.
Other relevant patent documents include U.S. Pat. No. 5,473,240 (the '240 patent) issued Dec. 5, 1995, to Moreira; U.S. Pat. No. 5,663,631 (the '631 patent) issued Sep. 2, 1997, to Kajiura et al.; U.S. Pat. No. 5,694,311 (the '311 patent) issued Dec. 2, 1997, to Umeda et al.; U.S. Pat. No. 5,719,486 (the '486 patent) issued Feb. 17, 1998, to Taniguchi et al.; and, U.S. Pat. No. 5,742,498 (the '498 patent) issued Apr. 21, 1998 to Taniguchi et al.
Briefly, the '240 patent describes a method and an apparatus for controlling operation of a brushless, permanent magnet motor, wherein signals containing third harmonic components of the motor flux density are acquired and filtered to isolate the third harmonic components. The filtered third harmonic components are then integrated to produce a time integral signal; the back EMF of one stator phase is sensed and the resulting EMF signal is filtered; and, zero crossings of the filtered signals are sensed and used to drive an inverter controller.
The '631 patent describes a full-wave-rectifying-circuit composed of six diodes and six transistors inversely connected in parallel with the diodes. A phase-control-circuit has three magnetic sensors, each of which corresponds to one of the phase-windings and is positioned so as to lag by an electric angle of .pi./2 radians to provide digital signals that lag by an electric angle of 2.pi./3 radians behind the induced-phase-signal. The six transistors are driven by the digital signals to form controlled AC voltages that lag behind the induced line voltages by an electric angle of 2.pi./3 radians so that the alternator increases the output power without increase of the body size.
The '311 patent describes a power supply system having a three-phase alternating current generator, a storage battery, and a three-phase rectifying device connected between the alternating current generator and the storage battery. The rectifying device includes three SiC-MOSFETS, which are turned on when the corresponding alternating voltage is positive and turned off when the corresponding alternating voltage is negative. A duty control device switches each of the three SiC-MOSFETS on and off according to a selected duty ratio so that the alternating current generator can generate an optimum voltage proportional to a rotational speed of a rotor of the alternating current generator.
The '486 patent describes a generating apparatus including a generator, first and second rectifiers, and a voltage regulator. The generator has three-phase star-connected armature windings, which generate three-phase high-output voltage at phase-terminals and low-output voltage at a neutral point. The first rectifier is a three-phase full-wave rectifier connected between the phase-winding and a high-voltage load, and the second rectifier is a diode connected between the neutral point and a low-voltage load with a battery. The regulator regulates the high output voltage to energize the high-voltage load and, at the same time, the low-output voltage to become an optimum voltage level for charging the battery. MOSFETs may be used for the first rectifier to short-circuit the armature windings, thereby supplying them with leading currents, which are respectively ahead of the high-output voltages, so as to increase the generator output power.
The '498 patent describes an alternator including MOSFET's used as rectifiers which are sealed by a sealing member (e.g., a molded epoxy resin) together with heat radiating fins and an external input/output terminal plate. The sealing member can evenly dissipate stress even if the radiating fins, terminal plate, and the like vibrate with different phases. Since the rectifying elements are reliably secured even when subjected to great vibrations, damage to the MOSFET's can be suppressed. The MOSFET's are never exposed to atmospheric air since the MOSFET's are sealed by the sealing member. Therefore, the alternator can improve the MOSFET's resistance against adverse operational conditions.
Although the systems, methods, and apparatuses mentioned above have been used for increasing alternator/generator output power, there is a need for an alternator system with increased output power that maintains a set battery voltage level, is both reliable and low-cost, and does not rely upon a plurality of sensors for controlling the output power.